1. Operating status of cross-linked cable joints
6-10KV high-voltage power cables are widely used in water conservancy projects and power systems. Their intact joints and accessories are very important for the safe, economical, reliable operation of electromechanical equipment and the safety of power supply. Cable joints with good design and reasonable construction can be used for a long time in most cases through actual operation. However, due to its strong current carrying capacity and high current density, cross-linked cables have stricter requirements for conductor connection quality. The mechanical and electrical conditions required for joints are becoming increasingly stricter and higher, especially for 6-10KV motor cables. Various joints will be subject to great thermal stress and short-circuit currents of higher severity and duration. Therefore, cross-linked cable accessories are not accessories, nor are they secondary components. They are equally important and essential components as the cables, and are also key products closely related to safe operation. Cross-linked cables have been widely used abroad. Although they have been widely used in China for only more than 10 years, there are still some problems. However, with the development of technology, the matching of accessories, the improvement of quality, and the perfection of technology, cross-linked cables have a tendency to replace oil-paper cables and have broad and far-reaching development prospects.
Cold shrink cable joint
2. Analysis of causes of cross-linked cable joint failures
Because there are many types, forms, and specifications of cable accessories; the quality is uneven; the technical level of construction personnel varies; and the operating modes and conditions of cable joints are different, the reasons for the failure of cross-linked cable joints are different. Due to the different media between cross-linked cables and oil-paper cables, the causes of joint failure are very different. The main cause of joint failure of oil-paper cables is the insulation effect, while the failure of cross-linked cable joints is mainly due to the conductor connection. Cross-linked cables are allowed to operate at high temperatures, which puts forward higher requirements for cable joints, making the problem of joint heating even more prominent. The contact resistance is too large, the temperature rise is accelerated, and the heat generation is greater than the heat dissipation, which causes the oxide film of the joint to thicken, which in turn makes the contact resistance larger and the temperature rise faster. Such a vicious cycle will destroy the insulation layer of the joint, form a short circuit between phases, and cause explosion and burning. The reasons for the increase in contact resistance are as follows.
1. Poor workmanship. Mainly refers to the construction technology of cable joint construction personnel before and after conductor connection.
(1) The contact surface of the connecting hardware is not properly processed. Whether it is a terminal block or a connecting tube, due to the conditions of production or storage, impurities, burrs and oxide layers often exist on the inner wall of the tube. This is a defect that people do not pay attention to, but it has a serious impact on the quality of conductor connections. In particular, a hard and insulating aluminum oxide film is easily formed on the surface of aluminum, making the connection of aluminum conductors more troublesome than the connection of copper conductors, and the process technology is much more stringent. The main cause of heat generation in connections (crimping, welding and mechanical connections), in addition to machine tools and material performance factors, is also the key to process technology and responsibility. If the construction workers do not understand the connection mechanism and do not strictly follow the process requirements, the connection will not reach the electrical and mechanical strength. Operation has confirmed that when the contact surface between the crimping fitting and the wire is cleaner, when the joint temperature increases, the oxide film produced becomes thinner and the contact resistance becomes smaller.
(2) Conductor damage. The strength of the cross-linked insulation layer makes it difficult to peel and cut. When cutting, the construction workers use an electrician's knife to cut left and right, and sometimes simply use a hacksaw to cut deep marks. Often, the control is not done well and the wires are damaged. Although the stripping is not serious, when the wire core is bent and the crimping creeps, the damaged conductor will be damaged or broken. It is not easy to find after the crimping is completed, and serious heating will occur due to the reduction in cross-section.
(3) The wire core is not in place when the conductor is connected. When connecting conductors, the insulation stripping length requires the hole depth of the crimping fitting to be increased by 5mm. However, because the hole depth of the product is not standard, the stripping length may not be enough, or a gap will be formed at the end of the wire due to the string position during crimping. The wall thickness of the fitting alone will lead to an increase in contact resistance and heat generation.
Cold shrink cable head
2. Not enough pressure. The current relevant information only mentions the number of crimps at each end when connecting cables in the joint manufacturing process and standard drawings, but does not detail the crimping area and crimping depth. The construction workers pressed the required number of pits, but the effect cannot be determined. No matter what form of pressure connection it is, the joint resistance is mainly the contact resistance, and the size of the contact resistance is related to the size of the contact force and the actual contact area, and is related to the output tonnage of the crimping tool used. The main reasons for insufficient conductor connection pressure are the following three points.
(1) The pressure of the crimping machine is insufficient. In recent years, there have been many manufacturers of crimping machines, and their management is chaotic. There is no unified standard. Especially for the mechanical crimpers produced in recent years, the crimping pit is not only narrow, but also the upper and lower dies cannot match after the crimping is in place. There are also some manufacturers who purchase or produce foreign types of crimping pliers. Since they implement foreign standards, they are not suitable for the nominal cross-section of domestic wires, so it is difficult to guarantee the crimping quality.
(2) There are large gaps in the connecting hardware. Nowadays, the connecting hardware used by most units of cross-linked cable joints is still the terminals and crimped tubes produced by oil-paper cables based on fan-shaped conductors. Theoretically speaking, the effective cross-sections of round and fan-shaped wire cores are the same, but in actual operation, the crimping effects of the two are quite different. Since the cross-linked cable conductor is a tightly twisted round core, there is a large gap in the inner diameter of the commonly used fittings and cannot achieve sufficient compression force after crimping. Contact resistance is inversely proportional to applied pressure and will therefore cause an increase.
(3) Fake and shoddy products are of poor quality. Fake and inferior hardware not only has impure materials and rough appearance, and is prone to cracks after pressing, but also has inaccurate specifications. The effective cross-section is very different from the genuine product, which cannot meet the crimping quality requirements at all. It will generate serious heat during normal operation and will inevitably malfunction if the load fluctuates slightly. 3. Insufficient cross-section: Comparing the allowable carrying capacity of cross-linked cables and oil-paper cables when the ambient temperature is 25°C, the conclusion is that ZQ2-3×240 oil-paper copper core cables can be replaced by YJV22-3×150 cross-linked copper core cables. Because the permitted carrying capacity of YJV22-3×150 cross-linked cable is 476A; while the permitted carrying capacity of ZQ2-3×240 oil-paper cable is 420A, which is more than 56A. ZLQ2-3×240 can be replaced by YJLV22-31×50, because the carrying capacity of the cross-linked 3×150 aluminum core cable is 364A, while the carrying capacity of the oil-paper 3×240 aluminum core cable is only 320A, and the cross-linked cable exceeds 44A. If calculated based on the allowable carrying capacity, 150mm2 cross-linked cables are basically the same as 240mm2 oil-paper cables, or 150mm2 cross-linked cables must be connected with 240mm2 hardware to operate normally. It can be seen that insufficient cross-section of the connecting hardware will be an important reason for serious heating of cross-linked cable joints. 4. Poor heat dissipation. For wrapped joints and various cast-type joints, not only the wrapping insulation is thicker than the cross-linked insulation layer of the cable, but also a mixture is injected into the shell. It is the smallest type of heat shrink joint, and its insulation and protective layer are more than double that of the cable body. In this way, no matter what type of joint, there is difficulty in heat dissipation. The current insulation materials of various joints have poor heat resistance. The normal working temperature of J-20 rubber self-adhesive tape does not exceed 75℃; J-30 only reaches 90℃; the use conditions of heat shrinkable materials are -50~100℃. When the cable is running under normal load, the temperature inside the joint can reach 100°C. When the cable is fully loaded, the temperature of the cable core wire reaches 90°C, and the joint temperature will reach about 140°C. When the temperature rises again, the oxide film at the joint thickens, and the contact resistance increases accordingly. Under the action of a certain power-on time, the insulating material of the joint carbonizes into non-insulating material, leading to failure. In summary, increasing the pressure on the contacts of the connecting hardware, lowering the operating temperature, cleaning the surface of the connecting metal material, improving the structural dimensions of the connecting hardware, selecting high-quality standard accessories, and strict construction technology are several key factors to reduce contact resistance.
European cable connector
3. Countermeasures to improve the quality of cross-linked cable joints
Due to the different environments and operating modes of cross-linked cable joints, as well as the different connected electrical equipment and locations, cable accessories have a lot of room for choice in terms of materials, structures and installation processes. However, the basic performance of all types of accessories is the same, so the following measures should be strengthened to improve the quality of the joints.
(1) Cable accessories must be selected with advanced technology, mature technology, reliable quality, and adaptability to the environment and conditions used. We must resolutely resist counterfeit and shoddy products, focus on testing new technologies, new processes, and new products, continuously summarize and improve them, and gradually promote and apply them year by year.
(2) Use connecting hardware with excellent materials, specifications and cross-sections that meet the requirements, and can operate safely and reliably. For wiring terminals, the oil-blocking type should be used as much as possible, because this type of terminal generally has a larger cross-section, which can reduce heat generation and can effectively solve the moisture-proof seal. The connecting pipe should be machined from copper rods or 1# aluminum, and the specifications and dimensions should match the core diameter of the cross-linked cable. In recent years, the crimping tubes for cross-linked cable joints provided by the accessory factories of Changsha and Shenyang Cable Factory have achieved good results.
(3) Choose a crimping machine with large crimping tonnage, good mold fit, sufficient pressure pit area, and a crimping effect that can meet the technical requirements. Prepare the interface before crimping and apply conductive paste.
(4) Train cable technicians who are technically competent, skilled in workmanship, serious and responsible in their work, and capable of cable construction, installation, operation and maintenance. Improve construction personnel's familiarity with cross-linked cables, enhance their understanding of the characteristics of cross-linked cable accessories, research technology, improve processes, formulate construction specifications, strengthen quality control, and ensure safe operation. Due to the short time of popularization and application of cross-linked cables, the variety of cable accessories, and the technical level of construction personnel, and the contact force and actual contact area of the joints change with the various operating conditions of the joints during operation, the reasons for the failure of various joints of cross-linked cables are also different. In addition to heating problems, joint failures caused by sealing problems, stress problems, connection problems, grounding problems, etc. should also be paid attention to.
Cable plug
